Retroreflectivity refers to a physical property of reflecting light back in an incident direction of the light and materials having retroreflectivity are generally used for road signs and roadway markings to enhance visibility while promoting driver safety. Excellent retroreflectivity means that a large amount of incident light is reflected toward an incident direction of the light. Retroreflectivity may be affected by surface area characteristics, a composition of materials, and refractive index and transparency of the materials in combination. Retroreflective materials have generally been developed based on silicon materials having excellent price competiveness and optical properties. The retroreflective materials have various shapes, such as a bead shape, a prism shape, a plane shape, a combined shape, and the like.
Korean Application No. 10-2010-0009432 discloses a composite retroreflective material prepared by bonding a plurality of a small glass bead around a large glass bead using a light reflective adhesive. This technology provides a high reflective index composite, which reflects incident light in multiple directions due to irregular reflection property, and because the multiple directions include an incident direction of light, it demonstrates partial retroreflectivity. However, the retroreflectivity of the composite based on irregular reflection is insufficient in terms of an efficiency of retroreflectivity. This technology has drawbacks that the refractive properties of the composite significantly change depending on various circumstances such as rainy conditions and the like, manufacturing costs are increased also due to the characteristic complexity of the composite structures and that the mass production is inhibited, thereby limiting commercialization of the composite.
U.S. Pat. No. 7,045,475 discloses retroreflective glass beads having refractive index (nd) of 1.59 or more, prepared from a composition including silicone oxide. This technology aims to promote retroreflectivity using glass beads having a uniform spherical shape, and thus the beads are not structurally complicated. However, since refractive index (nd) of 1.59 or more is not sufficient to realize retroreflectivity using the spherical glass beads, it is not possible to provide excellent retroreflectivity. Further, in the case of providing various colors to the glass beads through addition of colorants such that the glass beads can be applied to various fields including road signs, retroreflective sheets and the like, the colorants can cause the negative effects such as devitrification or reduction in refractive index. For example, in the case of retroreflective sheets requiring addition of colorants according to colors of fabric materials and entailing a brightness changing phenomenon due to an interfacial effect between the glass beads and the fabric materials, it is difficult to maintain transparency and refractive index of the glass beads while reducing brightness of the glass beads.
Due to such technical and practical limitations, there has been a continuous need for retroreflective glass beads as retroreflective materials, which have the following properties: 1) a simple spherical structure with high durability, 2) retroreflectivity maintained under various circumstances including the rainy or dry conditions and the like, 3) various optical properties including transparency, refractive index and the like, which are carefully combined to ensure retroreflectivity with increased reflection of incident light back in a direction of an incident angle, and 4) no devitrification, reduction in refractive index, and the like, even in the case of adding colorants. However, until now, glass beads having excellent retroreflectivity and high refractive index satisfying such requirements have not been developed.